PROBLEM PASSING ARRAY OF STRUCTURE TO VC++ DLL

  • Thread starter Thread starter Phoenix
  • Start date Start date
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Phoenix

Hi Friends ,

Could anyone please help me to resolve the following issue :

I pass an array of structures to a dll written in VC++ 6.0 whih fills
it with data . The following works well for VB 6.0 but when I wrote
the same piece of code in VB 2005
the array of structures remained empty even after the API call :

I have a structure as follows :

Structure tree_struct
Dim level As Short
Dim branch_id As String
Dim s_desc As String
Dim parent_id As String
Dim access As Short
Dim flag As Short
Dim branch_desc As String
End Structure

My declaration statement looks like following :

Declare Function fetch_tree_hierarchy Lib "mydll.dll" (ByVal
tree_handle As Short, ByVal app_id As String, ByRef l_tree_struct() As
tree_struct) As Short

Following is a sub written in VB 2005 which calls the above function
"fetch_tree_hierarchy " present in a dll "mydll.dll" written in vc++
6.0 :



Dim i As Integer = 0

Dim l_tree_struct() As tree_struct
ReDim l_tree_struct(100)

retcode = vb_get_menu_hierarchy(tree_handle, "app_id",
l_tree_struct)

If retcode = SUCESS Then

Console.WriteLine("Get Tree Hierarchy Succeeded")

Else

GetErrorText(retcode) 'Is a function which maps error
codes to error text
Call terminate_all()
Exit Sub

End If

In above case the Array of structure is coming empty.

Some Progress , Still No Results

During my exploration of various possibilities I went through an
example given at Microsoft website and did the following changes to
my above code :


<StructLayout(LayoutKind.Sequential, CharSet:=CharSet.Ansi)> Public
Structure tree_struct
Dim level As Short
Dim branch_id As String
Dim s_desc As String
Dim parent_id As String
Dim access As Short
Dim flag As Short
Dim branch_desc As String
End Structure


Public Class LibWrap
Declare Function fetch_tree_hierarchy Lib
"tcvbodss.dll" (ByVal mnu_handle As Short, ByVal apl_id As String,
ByRef outArray As IntPtr) As Short
End Class



Dim outArray As IntPtr
LibWrap.vb_get_menu_hierarchy(tree_handle, "app_id",
l_tree_struct)
Dim manArray(100) As vb_menu
Dim current As IntPtr = outArray
Dim i As Integer

For i = 0 To 100 - 1

manArray(i) = New tree_struct

Marshal.PtrToStructure(current, manArray(i))

Marshal.DestroyStructure(current, GetType(tree_struct))

current = IntPtr.op_Explicit(current.ToInt64() +
Marshal.SizeOf(manArray(i)))

Console.WriteLine("Element {0}: {1} {2}", i,
manArray(i).branch_id, manArray(i).branch_desc)

Next i

Marshal.FreeCoTaskMem(outArray)

At line : " Marshal.PtrToStructure(current, manArray(i))" I am getting
following exception :

Message : Value cannot be null.
Parameter name: ptr

My mistake may be quite silly as I am new to VB 2005 , kindly help me
to resolve this problem.

Sincerely,

Sudhansu
 
I pass an array of structures to a dll written in VC++ 6.0 whih fills
it with data . The following works well for VB 6.0 but when I wrote
the same piece of code in VB 2005
the array of structures remained empty even after the API call :
Click to expand...


Can you post the working VB6 code so we can compare?


Mattias
 
Can you post the working VB6 code so we can compare?

Mattias
Click to expand...
Hi Mattias ,

Thanks a lot for the reply , following is the working VB 6 code for
your reference :

Type tree_struct
Dim level As Integer
Dim branch_id As String
Dim s_desc As String
Dim parent_id As String
Dim access As Integer
Dim flag As Integer
Dim branch_desc As String
End Type

My declaration statement looks like following :

Declare Function fetch_tree_hierarchy Lib "mydll.dll" (ByVal
tree_handle As Integer, ByVal app_id As String, ByRef l_tree_struct()
As tree_struct) As Integer


Dim retcode As Integer

Dim l_tree_struct(100) As tree_struct

retcode = fetch_tree_hierarchy(tree_handle, "app_id",
l_tree_struct)

Sincerely,

Sudhansu
 
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